Properties of Ionic and Covalent Compounds

If you know the chemical formula of a compound, you can predict whether it contains ionic bonds, covalent bonds, or a mixture of bond types. Nonmetals bond to each other via covalent bonds while oppositely charged ions, such as metals and nonmetals, form ionic bonds. Compounds which contain polyatomic ions may have both ionic and covalent bonds.

Identifying Bond Types

But, how do you know if a compound is ionic or covalent just by looking at a sample? This is where the properties of ionic and covalent compounds can be useful. Because there are exceptions, you need to look at several properties to determine whether a sample is ionic or covalent, but here are some characteristics to consider:

• Crystals: Most crystals are ionic compounds. This is because the ions in these compounds tend to stack into crystal lattices to balance between the attractive forces between opposite ions and the repulsive forces between like ions. Covalent or molecular compounds can exist as crystals, though. Examples include sugar crystals and diamond.
• Melting and boiling points: Ionic compounds tend to have higher melting and boiling points than covalent compounds.
• Mechanical properties: Ionic compounds tend to be hard and brittle while covalent compounds tend to be softer and more flexible.
• Electrical conductivity and electrolytes: Ionic compounds conduct electricity when melted or dissolved in water while covalent compounds typically don't. This is because covalent compounds dissolve into molecules while ionic compounds dissolve into ions, which can conduct charge. For example, salt (sodium chloride) conducts electricity as molten salt or in salt water. If you melt sugar (a covalent compound) or dissolve it on water, it won't conduct.

Examples of Ionic Compounds

Most ionic compounds have a metal as the cation or first part of their formula, followed by one or more nonmetals as the anion or second part of their formula. Here are some examples of ionic compounds:

• Table salt or sodium chloride (NaCl)
• Sodium hydroxide (NaOH)
• Chlorine bleach or sodium hypochlorite (NaOCl)

Examples of Covalent Compounds

Covalent compounds consist of nonmetals bonded to each other. These atoms have identical or similar electronegativity values, so the atoms essentially share their electrons. Here are some examples of covalent compounds:

• Water (H₂O)
• Ammonia (NH₃)
• Sugar or sucrose (C₁₂H₂₂O₁₁)

Why Do Ionic and Covalent Compounds Have Different Properties?

The key to understanding why ionic and covalent compounds have different properties from each other is understanding what's going on with the electrons in a compound. Ionic bonds form when atoms have different electronegativity values from each other. When the electronegativity values are comparable, covalent bonds form.

But, what does this mean? Electronegativity is a measure of how easily an atom attracts bonding electrons. If two atoms attract electrons more or less equally, they share the electrons. Sharing electrons results in less polarity or inequality of charge distribution. In contrast, if one atom attracts bonding electrons more strongly than the other, the bond is polar.

Ionic compounds dissolve in polar solvents (like water), stack neatly on each other to form crystals, and require a lot of energy for their chemical bonds to break. Covalent compounds can be either polar or nonpolar, but they contain weaker bonds than ionic compounds because they are sharing electrons. So, their melting and boiling points are lower and they are softer.

Sources

• Bragg, W. H.; Bragg, W. L. (1913). "The Reflection of X-rays by Crystals". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 88 (605): 428–438. doi:10.1098/rspa.1913.0040
• Langmuir, Irving (1919). "The Arrangement of Electrons in Atoms and Molecules". Journal of the American Chemical Society. 41 (6): 868–934. doi:10.1021/ja02227a002
• McMurry, John (2016). Chemistry (7th ed.). Pearson. ISBN 978-0-321-94317-0.
• Sherman, Jack (August 1932). "Crystal Energies of Ionic Compounds and Thermochemical Applications". Chemical Reviews. 11 (1): 93–170. doi:10.1021/cr60038a002
• Weinhold, F.; Landis, C. (2005). Valency and Bonding. Cambridge. ISBN 0-521-83128-8.